The invention relates to a plasma particulate filter.
DE 100 57 862C1 discloses a plasma particulate filter and a method for reducing the levels of carbon-containing particulate emissions from diesel engines, in which the particulates contained in the exhaust gas are deposited on filter surfaces, the deposited particles being oxidized in order to regenerate the filter, and the regeneration being effected by non-thermal, electrical sliding surface discharges at the surfaces covered with particulates.
DE 100 57 862 C1 has described various geometries for operating an arrangement of this type which are based on the principle of what are known as wall flow filters. These filters comprise parallel passages with a quadrilateral cross section which are alternately closed on the outlet side and the inlet side of the exhaust gas. This results in a division into inlet passages for the particulate-laden exhaust gas and outlet passages for the filtered exhaust gas. The particulates are deposited on the inner walls of the passages that are open on the inlet side and are oxidized there by oxygen and hydroxyl radicals which are produced in the immediate vicinity of the wall by non-thermal sliding surface discharge plasmas.
DE 100 57 862 C1 works on the basis that an electrode be arranged at each of the edges of a filter passage in order to produce sliding surface discharges. The electrodes required to produce plasma can either be embedded in the filter material or applied to the filter material, in such a way that in any event there is a layer with a high dielectric strength between an electrode connected to high voltage and the counterelectrode that is connected to ground. The embedding of the electrodes described in that document, however, means that sliding surface discharges can only be generated on both sides of the cell walls, whereas the particulates are only deposited on one side. This means that the specific energy consumption for the regeneration is twice as high as is actually necessary.
On the other hand, electrodes which are exposed to the exhaust gas and are proposed in that document in combination with embedded electrodes for the preferential operation of sliding surface discharges on one side of the wall, on account of being in contact with the exhaust gas are exposed to erosion processes which may be boosted still further by gas discharge processes. These erosion processes may not only have an adverse effect on the service life of the electrodes in particular, but also, via the formation of metal oxides, on the service life of the ceramic.
A further drawback is that the large number of electrodes—specifically four per inlet passage—significantly increases the size and weight of the plasma particulate filter compared to known filters.
The literature has disclosed geometries for the operation of dielectric barrier discharges in ceramic honeycomb bodies (cf. for example EP 0 840 838 B1), in which a cylindrical volume which includes a large number of passages could be excited by an internal high-voltage electrode and an external ground electrode. However, this means that it is not possible to differentiate between inlet and outlet passages of a particulate filter and also it is impossible to produce targeted sliding surface discharges. Moreover, the long sparking distance between the electrodes means that a high voltage amplitude of 20 kV is required, which can lead to problems in the motor vehicle.